Strip loaded waveguide with low-index transition layer

ABSTRACT

A strip loaded waveguide comprises a slab and a strip, wherein the strip is separated from the slab. Nevertheless, a guiding region is provided for propagating an optical mode and this guiding region extends both within the strip and the slab. A layer of material having an index of refraction lower than that of the strip and the slab may be disposed between and separate the strip and the slab. In one embodiment, the slab comprises a crystalline silicon, the strip comprises polysilicon or crystalline silicon, and the layer of material therebetween comprises silicon dioxide. Such waveguides may be formed on the same substrate with transistors. These waveguides may also be electrically biased to alter the index of refraction and/or absorption of the waveguide.

PRIORITY APPLICATION

This application claims priority under 35 U.S.C. § 119(e) from U.S.Provisional Patent Application Ser. No. 60/318,456, entitled “StripLoaded Waveguide with Low-Index Transition Layer” and filed Sep. 10,2001 as well as U.S. Provisional Patent Application Ser. No. 60/318,445entitled “SOI Waveguide with Polysilicon Gate” and filed Sep. 10, 2001,both of which are hereby incorporated by reference herein in theirentirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention is directed to waveguides, and more particularly,to waveguides formed on a substrate.

2. Description of the Related Art

Light offers many advantages when used as a medium for propagatinginformation, the foremost of which are increased speed and bandwidth. Incomparison with electrical signals, signals transmitted optically can beswitched and modulated faster and can include an even greater number ofseparate channels multiplexed together. Accordingly, lightwavetransmission along optical fibers is widespread in thetelecommunications industry. In an exemplary fiber optic communicationsystem, a beam of light may be emitted from a laser diode and modulatedusing an electro-optical modulator that is driven by an electricalsignal. This electrical signal may correspond to voice or data which isto be transmitted over a distance between, e.g., two components in acomputer, two computers in a network, or two phones across the countryor the world. The light travels in an optical fiber to a location whereit is detected by an optical sensor which outputs voltage that varies inaccordance with the modulation of the optical beam. In this manner,information can be rapidly transported from one location to another.

Accordingly, various components have been developed to process andmanipulate optical signals. Examples of such components includemodulators, switches, filters, multiplexers, demultiplexers to name afew. Other useful optical components include lasers and opticaldetectors as well as waveguides. Many of these components can be formedon a substrate. It is therefore highly desirable to combine a variety ofsuch components into a system that is integrated onto a singlesubstrate. In such a system, optical waveguides theoretically could beused to propagate optical signals between components on the substrate.

SUMMARY OF THE INVENTION

One aspect of the present invention comprises a strip loaded waveguidecomprising a slab portion having a first refractive index n₁, a stripportion having a second refractive index n₂, and a transition portionbetween the slab portion and the strip portion. The transition portionhas a refractive index n₃ that is less than the first refractive indexn₁ and the second refractive index n₂.

Another aspect of the present invention comprises a strip loadedwaveguide comprising a slab portion and a strip portion. The stripportion is disposed with respect to the slab portion to form a guidingregion. A first portion of the guiding region is in the strip portion,and a second portion of the guiding region is in the slab portion. Theguiding region propagates light in a single spatial mode and only in atransverse electric mode.

Another aspect of the present invention comprises a strip loadedwaveguide comprising a slab portion and a strip portion. The stripportion is disposed with respect to the slab portion to form a guidingregion. A first portion of the guiding region is in the strip portion,and a second portion of the guiding region is in the slab portion. Theguiding region propagates light in a single spatial mode with across-sectional power distribution profile having two intensity maxima.A first intensity maxima is located in the slab portion, and the secondintensity maxima is located in the strip portion.

Another aspect of the present invention comprises a waveguide having aguiding region for guiding light through the waveguide. The guidingregion comprises a layer of polycrystalline silicon juxtaposed with alayer of crystal silicon.

Yet another aspect of the present invention comprises an apparatuscomprising a strip loaded waveguide, a transistor, and a substrate. Thestrip loaded waveguide comprises a slab portion having a firstrefractive index n₁, a strip portion having a second refractive indexn₂, and a transition layer between the slab portion and the stripportion. The transistor comprises first and second portions and adielectric layer therebetween. The dielectric layer of the transistorand the transition layer of the waveguide comprise the same material.The substrate supports both the transistor and the waveguide.

Yet another aspect of the present invention comprises an apparatuscomprising a strip loaded waveguide, a transistor, and a substrate. Thestrip loaded waveguide comprises a slab portion having a firstrefractive index n₁ and a strip portion having a second refractive indexn₂. The transistor comprises first and second portions and a dielectriclayer therebetween. The second portion of the transistor and the slabportion of the waveguide are formed of a single layer of material. Thesubstrate supports both the transistor and the waveguide.

Still another aspect of the present invention comprises a method ofchanging the index of refraction of a strip loaded waveguide comprisinga semiconductor slab and a conductive strip that are separated by aninsulating layer. The method comprises dynamically changing the carrierdistribution in the semiconductor slab.

Still another aspect of the present invention comprises a waveguideapparatus. The waveguide apparatus comprises a slab portion having afirst refractive index, a strip portion having a second refractiveindex, and a transition portion between the slab portion and the stripportion. The transition portion has a third refractive index that isless than the first refractive index and the second refractive index.The waveguide apparatus additionally comprises a voltage sourceconfigured to apply a voltage between the strip portion and the slabportion such that an electric field is introduced between the stripportion and the slab portion.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention are described below inconnection with the accompanying drawings, in which:

FIG. 1 is a schematic illustration of a generic subsystem comprising aplurality of components connected together via optical waveguides;

FIG. 2 is a perspective cutaway view of a strip loaded waveguidecomprising a slab having a relatively high refractive index, a stripalso having a relatively high refractive index formed on the slab, and atransition layer having a relatively low refractive index positionedbetween the slab and the strip;

FIG. 3 is a cross-sectional view of a strip loaded waveguide furtherincluding a map of an exemplary magnetic field distributioncorresponding to the fundamental mode supported by the strip loadedwaveguide;

FIG. 4 is a plot on axes of intensity (in arbitrary units) and position,Y, (in arbitrary units) juxtaposed adjacent a cross-sectional view ofthe strip loaded waveguide showing the optical intensity profile of thefundamental within the waveguide structure;

FIG. 5 is a cross-sectional schematic illustration of a strip loadedwaveguide and a transistor fabricated on the same substrate;

FIG. 6 is a cross-sectional schematic illustration of a strip loadedwaveguide including gate spacers;

FIG. 7 is a cross-sectional schematic illustration of a strip loadedwaveguide configured to be biased electronically; and

FIG. 8 is a cross-sectional schematic illustration of an alternativestrip loaded waveguide configured to be biased electronically so as toalter the index of refraction predominately within the strip.

FIG. 9 is a cross-sectional schematic illustration of a strip loadedwaveguide comprising a polysilicon strip on a crystal silicon slab andnot including a low-index transition layer therebetween.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One preferred embodiment of the present invention comprises anintegrated optical subsystem formed on a substrate. Such subsystems maybe part of a larger optical system which may or may not be formed on asingle substrate. FIG. 1 illustrates a generic integrated opticalsubsystem 140 formed on the surface of substrate 130. The substrate 130may serve as a platform for the integrated optical subsystem 140, andthus preferably comprises a volume of material of sufficient thicknessto provide physical support for the integrated optical subsystem 140.This substrate preferably comprises a material such as silicon orsapphire.

In the embodiment illustrated in FIG. 1, a plurality of components 100are connected by one or more integrated optical waveguides 110 and asplitter 120. The components 100 may comprise optical components,electronic components, and optoelectronic or electro-optic components.The optical and electro-optical components may include waveguide devicesor non-waveguide devices, i.e., light may propagate through suchcomponents and be guided or unguided. Examples of optical,electro-optic, and optoelectronic components include, but are notlimited to, light sources, detectors, modulators, reflectors,polarizers, phase shifters, filters, and mode-converters.

The integrated optical waveguides 110 may be arranged in anyconfiguration to connect components 100 as desired for a particularapplication. For example, an optical signal from a one component can betransmitted to a plurality of other components through the use ofsplitter 120, as shown. The variety of configurations of waveguides andcomponents is unlimited. Waveguides can follow different paths and canbend and turn, split, cross, and can be combined. Different components,electrical, optical, electro-optic, and optoelectronic can be includedon the substrate, and in various embodiments, can be optically coupledto the waveguides and to each other. In addition, electrical connectionscan be made to the components and to the waveguides as is discussed morefully below. The arrangement of waveguides and components is not to beconsidered limited but may include any variety of combinations andjuxtapositions.

In some embodiments, the substrate 130 will support a plurality ofmaterial layers which together create layers of integrated opticalsubsystems stacked atop each other. Each of these layered integratedoptical subsystems may include waveguides and/or components, electrical,optical, electro-optic, and optoelectronic, formed within a given layer.Such multi-layered stacking will add to the variety of integratedoptical designs that are possible. Light can be directed between thevarious layers using waveguides situated therebetween, gratings such asfor example waveguide gratings, and Bragg diffaction elements such asdistributed Bragg gratings. Multilayer optical films such as thin filmfilters can be incorporated to introduce the desired phase delay and maybe used to enable various functionalities, such as for example opticalfiltering. The structures and methods involved in coupling light fromone layer to another, however, are not limited to those recited herein.

In general, optical waveguides comprise a core region comprisingmaterial that is at least partially transparent. This core region issurrounded by a cladding region that confines light within the coreregion. Some optical energy, often referred to as the evanescent energyor the evanescent field, however, may exist outside the core region andwithin the cladding region.

In certain waveguides, the core region comprises a first material havinga first refractive index, and the cladding region comprises a secondmaterial having a second refractive index, the refractive index of thecore region being greater than the refractive index of the claddingregion. A core/cladding interface is located at the boundary between thecore region and the cladding region. In such embodiments, when light inthe core region is incident upon this core/cladding interface at anangle greater than the critical angle, the light is reflected back intothe core region. This effect is referred to as total internalreflection. In this manner, optical signals can be confined within thecore region due to total internal reflection at the core/claddinginterface.

Waveguides can be fabricated in a wide variety of geometries andconfigurations. An optical fiber is a specific type of waveguide thatfits the description above. An optical fiber generally comprises acircularly cylindrical core surrounded by an circularly cylindrical orannular cladding layer. The core has a relatively high refractive indexand the cladding has a relatively low refractive index. The core andcladding may comprise, e.g., silica or silica based materials, and aretypically flexible, with core diameters of approximately 10 μm forsingle-mode fiber. As discussed above, optical fibers are often used totransmit optical signals across large distances, ranging for examplefrom centimeters to thousands of kilometers.

Optical fibers should be distinguished from integrated opticalwaveguides, which are generally associated with a substrate. Theintegrated optical waveguide may for example be situated on thesubstrate, in a substrate, or partially on and partially in thesubstrate. The integrated optical waveguide may be part of the substrateitself but preferably comprises of one or more layers of materialpositioned on a surface of the substrate. Examples of integrated opticalwaveguides include channel waveguides, rib or ridge waveguides, slabwaveguides, and strip loaded waveguides, all of which are well-known inthe art. In contrast to optical fibers, integrated optical waveguidesare less likely to have a circularly symmetric cross-section although intheory they can be circularly cylindrical. Additionally, integratedoptical waveguides are generally used to transmit optical signalsbetween locations on the substrate, and thus preferably have lengthsranging from microns to centimeters.

In accordance with conventional usage in the art, optical componentsthat are integrated onto a substrate with integrated optical waveguides,are collectively referred to herein as integrated optics. Such opticalcomponent may for example, process, manipulate, filter or otherwisealter or control optical signals propagating within the waveguides. Asdiscussed above, these components themselves may be waveguides thatguide light.

One of the simplest integrated optical waveguide configurations is theconventional slab waveguide. The slab waveguide comprises a thin, planarslab surrounded by cladding regions. The cladding regions may take theform of first and second (for example, upper and lower) cladding layerson either side of the slab. The two cladding layers need not comprisethe same material. In this simplified example, the slab may be planarwith substantially parallel planar boundaries at the interfaces with thefirst and second cladding layers. Generally, the slab has a higherrefractive index than either of the cladding layers. Light can thereforebe confined in one dimension (e.g., vertically) within the slab. In thisconfiguration of the slab waveguide, optical energy is not confinedlaterally to any portion of the slab, but extends throughout the slabdue to total internal reflection at the planar boundaries between theslab and the surrounding upper and lower cladding layers.

A strip loaded waveguide is formed by positioning a strip on the slab ofa slab waveguide. The slab and the strip located thereon may besurrounded on opposite sides by the first and second (e.g., upper andlower cladding layers). Preferably, the strip has a refractive indexthat is greater than that of either cladding layer, however, the indexof the strip is preferably approximately equal to that of the slab. Thepresence of the strip positioned on the slab induces an increase ineffective index of the slab in the region beneath the strip and inproximity thereto.

Accordingly, the region within the slab that is beneath the strip and inproximity thereto has a higher effective refractive index than otherportions of the slab. Thus, unlike the slab waveguide wherein opticalenergy propagates throughout the planar slab, the strip loaded waveguidesubstantially confines optical energy to the region of the planar slablayer under the high-index strip. In a strip loaded waveguide,therefore, an optical signal can be propagated along a path in the slabdefined by the region over which the high-index strip is placed on theslab. Thus, slab waveguides defining any number and variations ofoptical pathways, can be created by depositing one or more strips ontothe slab having the shape and orientation of the desired opticalpathways.

FIG. 2 is a schematic cutaway illustration of a preferred embodiment ofa strip loaded waveguide 200. The strip loaded waveguide 200 comprises aslab 205 having a first refractive index n₁ and a strip 210 having asecond refractive index n₂. In addition, the strip loaded waveguide 200has a transition layer 215 having a third refractive index n₃. Thetransition layer 215 is positioned between the slab 205 and the strip210, such that the slab 205 and the strip 210 do not directly contacteach other. The refractive index of the transition layer n₃ is less thanthe refractive index of the slab n₁ and the refractive index of thestrip n₂.

In certain embodiments of the invention, semiconductor materials used inconventional processes for fabrication of semiconductor microelectronicsare employed to create strip loaded waveguides. These materials include,but are not limited to, crystalline silicon, polysilicon and silicondioxide (SiO₂). In particular, in one preferred embodiment, the slab 210comprises single crystal silicon, the transition layer 215 comprisessilicon dioxide and the strip 210 comprises polysilicon, although inother embodiments, the strip 210 may comprise crystal silicon. Thecrystal silicon slab 215 and the polysilicon strip 210 may be doped, forexample, in cases where the slab 215 or the strip 210 are to beelectronically conductive. In applications where the slab 215 or thestrip 210 need not be electronically conductive, the slab 215 and thestrip 210 are preferably undoped to minimize absorption losses.

As is well known, single crystal silicon is used to fabricatesemiconductor microelectronics and integrated circuits (ICs), such asmicroprocessors, memory chips, and other digital as well as analog ICs,and thus single crystal silicon is well characterized and its propertiesare largely well understood. The term single crystal silicon is usedherein consistently with its conventional meaning. Single crystalsilicon corresponds to crystalline silicon. Single crystal silicon,although crystalline, may include defects such that it is not truly aperfect crystal, however, silicon having the properties conventionallyassociated with single crystal silicon will be referred to herein assingle crystal silicon despite the presence of such defects. The singlecrystal silicon may be doped either p or n as is conventional. Suchdoping may be accomplished, for example, by ion implantation.

Single crystal silicon should be distinguished from polysilicon or“poly”. Polysilicon is also used to fabricate semiconductormicroelectronics and integrated circuits. The term polysilicon or “poly”is used herein consistently with its conventional meaning. Polysiliconcorresponds to polycrystalline silicon, silicon having a plurality ofseparate crystalline domains. Polysilicon can readily be deposited forexample by CVD or sputtering techniques, but formation of polysliconlayers and structures is not to be limited to these methods alone.Polysilicon can also be doped p or n and can thereby be madesubstantially conductive. In general, however, bulk polysilicon exhibitsmore absorption losses in the near infrared portion of the spectrum thana similar bulk single crystal silicon, provided that the doping,temperature, and other parameters are similar.

As illustrated in FIG. 2, the strip loaded waveguide 200 is preferablylocated on a supporting structure 220 or substrate. The supportingstructure 220 serves to support the strip loaded waveguide 200 andpreferably comprises a material such as a silicon or sapphire substrate222. Additionally, the supporting structure 220 may also include acladding layer 224, which aids in confining optical energy within theslab portion 205. Accordingly, this layer 224 preferably has arefractive index that is low in comparison to the refractive index ofthe slab 205.

In one preferred embodiment, the supporting structure 220 comprises asilicon substrate 222 having a cladding layer 224 of silicon dioxideformed thereon. The silicon dioxide layer 224 on the silicon substrate222 with an index of approximately 1.5 serves as a lower cladding layerfor the slab 205 having an index of approximately 3.5. This siliconsubstrate 222 may comprise doped silicon and may be a commerciallyavailable silicon wafer used for fabricating semiconductor integratedcircuits. In other embodiments, the cladding layer 224 may comprisesilicon nitride. The index of refraction of silicon nitride isapproximately 1.9.

In alternative embodiments, wherein the supporting structure 220comprises a material other than silicon, the cladding layer 224 ofsilicon dioxide may not be present. For example, the slab 205 may restdirectly on a sapphire substrate 222. Processes for growing crystalsilicon on sapphire have been developed. In general, in these cases, thesupporting structure 220 preferably has an index of refraction lowerthan that of the slab 205. In other embodiments, an additional claddinglayer 224 may be formed on the these non-silicon substrates.

The slab 205 is therefore disposed either on the substrate 222 or on alayer 224 (preferably the cladding) formed over the substrate. Thiscladding layer 224 itself may be formed directly on the substrate 222 ormay be on one or more layers formed on the substrate 222. The slabportion 205 may span the substrate 222 or extend over only a portion ofthe substrate 222. As discussed above, the slab 205 preferably comprisessingle crystal silicon and has an index of refraction n₁ on average ofabout 3.5 and has a thickness t₁ preferably between about${\frac{\lambda}{6n}\quad {and}\quad \frac{\lambda}{4n}},$

and more preferably about $\frac{\lambda}{4n},$

where n is the index of refraction. This thickness, t₁, determines inpart the optical mode or modes supported by the strip loaded waveguideand depends partially on the geometry of the structure. In alternativeembodiments, the slab 205 may comprise materials other than singlecrystal silicon and may be doped or undoped and thus may have differentrefractive indices. The slab 205, however, preferably comprises crystalsilicon. Localized doping, such as used to create the source, drain, andchannel regions in a transistor, may affect the optical properties ofthe slab 205. The index of refraction in localized regions of the slabcan vary slightly due to doping by ion implantation.

In general, the strip 210 is disposed above and in a spaced-apartconfiguration with respect to the slab 205. The strip 210 may comprisedoped polycrystalline silicon having an index of refraction n₂ ofapproximately 3.5. In alternative embodiments, the strip 210 maycomprise doped single crystal silicon having an index of refraction n₂on average about 3.5. As discussed above, however, the strip may also beundoped and may comprise materials other than polysilicon or crystalsilicon although these materials are preferred. An example of one suchalternative material that may used to form the strip 210 is siliconnitride (Si₃N₄), which has an index of refraction n₃ of approximately1.9.

The dimensions of the strip 210 may vary and depend in part on theoverall composition and geometry of the waveguide. As with the slab 205,however, the size of the strip 210 determines in part the number ofmodes to be supported by the waveguide and the wavelength of thesemodes.

The transition layer 215 is positioned between the slab 205 and thestrip 210. This transition layer 215 may span the slab 205 asillustrated in FIG. 2 or extend over only a portion of the substrate205. Preferably, the refractive index of the transition layer 215 isless than the refractive index of the polysilicon strip 210 and thecrystalline silicon slab 205. In one preferred embodiment, thetransition layer 215 comprises silicon dioxide having an index ofrefraction n₃ of approximately 1.5.

In various embodiments, the transition layer 215 may include opticallyactive (i.e., gain inducing) material, such as erbium. Waveguidestructures that include an optically active gain inducing material inthe transition layer 215 can produce gain and amplify or regenerate thestrength of the optical signal propagating through the waveguide.Specialized components can be formed using these amplifying structures.

In certain embodiments, the thickness t₃ of the transition layer 215 isequal to the thickness of the gate oxide layer of transistors (notshown) positioned on the same substrate as the strip loaded waveguide200 and fabricated in the same process as the strip loaded waveguide200. The width of the transition layer 215 may be substantially equal tothe width w₂ of the strip 210, although in other embodiments, such asillustrated in FIG. 2, the width of the transition layer 215 is greaterthan the width w₂ of the strip 210.

In the waveguide structure illustrated in FIG. 2, the strip loadedwaveguide 200 is covered by one or more coatings 230, although thesecoatings are optional. Two coatings are shown in FIG. 2, one with anindex of refraction n₄ and another thereon with an index of refractionn₅. More or less coatings may be used and in other configurations thecoatings 230 can be excluded and replaced instead with air or vacuum.The optional nature of these coatings 230 is emphasized by depicting thecoating in phantom in FIG. 2. These coatings 230, however, are usefulfor protecting the strip loaded waveguide 200 from damage orinterference which may occur due to contact with other objects.Accordingly, the coatings 230 preferably completely covers the striploaded waveguide 200, although in other case, the coating may extendonly over portions of the strip 210 or slab 205.

The coatings 230 may also serve as a cladding layer, providingconfinement of optical energy within the slab 205 and the strip 210.Accordingly, the coatings 230 preferably have indices of refraction n₄,n₅ less than that of the slab 205 and the strip 210. The coatings 230may have an index or refraction equal to that of the low-indextransition layer 215 and may comprise the same material as the low-indextransition layer 215. Alternatively, the coatings 230 may have adifferent indices of refraction than the transition layer 215 and maycomprise different material. In multilayered integrated opticalstructures, the coatings 230 may serve as a substrate for second striploaded waveguide in a layer disposed above a first strip loadedwaveguide.

Accordingly, the coatings 230 preferably comprises a solid, possiblyelectrically insulating material, having a refractive index less thanthat of the slab 205 and the strip 210. The coatings 230 may, forinstance, comprise glass or silicon dioxide. Other materials and, morespecifically, other dielectrics may also be employed. Polymericmaterial, such as for example polyimide may be used in certainapplications.

Confinement of light within the slab 205 is provided because the slab205 has a higher refractive index than the layers above and below. Inone preferred embodiment, for example, light is confined within thesilicon slab 205 because the silicon slab 205 has a higher refractiveindex than the glass coatings 230 covering it. In addition, the siliconslab 205 has a higher index than the silicon dioxide cladding layer 224immediately below it.

The light within the slab 205 is confined to portions beneath the strip210 because of the presence of the strip 210, despite the fact that thestrip 210 is separated from the slab 205. The intervening transitionlayer 215 does not prevent the strip 210 from determining the shape andlocation of the optical mode(s) supported in the slab 205. The presenceof the strip 210 positioned proximally to the slab portion 205 inducesan increase in effective index of the slab portion 205 in the regiondirectly under the strip 210 and in proximity thereto. This increase ineffective index defines a relatively high effective index guiding region225 wherein light in one or more supported optical modes is guided alongthe strip loaded waveguide 200. The strip loaded waveguide 200 guidessupported modes in the guiding region 225 despite the presence of thetransition layer 215 between the slab 205 and strip 210. In particular,the transition layer 215 does not prevent the strip 210 from alteringthe effective index within the slab 205 and more particularly, fromraising the effective index within the slab 205. Preferably, thetransition layer 215 has a thickness sufficiently small such that thestrip 210 can increase the effective index of the slab 205 in regionsimmediately beneath and in the proximity thereto. The transition layer215 is sufficiently thin and the strip 210 and the slab 205 aresufficiently close, although physically separated by the interveningtransition layer, that the strip 210 can affect the propagation of lightwithin the slab 205. The transition layer 215 also preferably has anindex of refraction that is low in comparison with that of the strip 210and the slab 205.

The guiding region 225 corresponds to a boundary where a specificportion of the optical energy within the mode, preferably thefundamental mode, is substantially contained and thus characterizes theshape and spatial distribution of optical energy in this mode.Accordingly, the guiding region 225 corresponds to the shape andlocation of the optical mode or modes in this strip loaded waveguide200. In the guiding region 225, the electric field and the opticalintensity are oscillatory, where as beyond the guiding region 225, theevanescent field exponentially decays.

Propagation of an optical signal in the strip loaded waveguide 200illustrated in FIG. 2 is further characterized by the spatialdistribution of the field strength across the cross-section of the striploaded waveguide 200. FIG. 3 illustrates the magnetic field distributionacross a cross-section of the waveguide 200 parallel to the x-y plane.This distribution is the result of modeling using finite difference timedomain iterations to calculate the horizontal component of the magneticfield in the mode supported by the structure, i.e., the fundamentalmode. The electric field is vertically polarized in this example. Thecase where the transition layer has the same refractive index as theregion surrounding the slab was modeled. As shown, the field strengthwithin the fundamental mode is distributed within the slab 205 despitethe presence of the transition layer 215 and the separation between thestrip 210 and the slab 205. The field, however, is localized within thestrip 210 and in the slab 205 within a region proximal to the strip.This field strength distribution is consistent with the guiding region225 shown in FIG. 2.

A schematic diagram of the intensity through the thickness of thewaveguide structure is presented in FIG. 4. This plot shows the opticalenergy substantially confined within the strip 210 and the region of theslab 205 below and adjacent to the strip 210.

The intensity profile shown in FIG. 4 is characterized by the presenceof a localized intensity minima 235 in the lowest-order guided mode. Thelocalized intensity minima 235 occurs in the proximity of the transitionlayer 215 between the strip 210 and the slab 205. Accordingly, thislocalized minima 235 is likely caused by the presence of the transitionlayer 215 and the separation of the slab 205 from the strip 210.Nevertheless, the presence of the transition layer 215 does notsubstantially disrupt the mode. Optical energy can still be propagatedalong a guiding region 225 partially within the strip 210 and the slab205. Accordingly, the propagation of light can be controlled and beamscan be directed along pathways defined by these strip loaded opticalwaveguides 200. Integrated optical systems can therefore be constructedwherein light is guided to and from components and thereby manipulatedand processed as desired.

Such integrated optical systems can be fabricated using waveguidessimilar to those disclosed herein. It will be appreciated that, althoughthe strip loaded waveguide 200 illustrated in FIG. 2 has a substantiallystraight configuration, it will be understood that in alternativeembodiments, the strip loaded waveguide can have an unlimited variety ofalternative configurations and orientations, including but not limitedto bends and turns, and intersections with other strip loadedwaveguides. See, e.g., FIG. 1. Additionally, although the strip loadedwaveguide 200 illustrated in FIG. 2 has a rectangular cross-section(parallel to in the x-y plane), other cross-sectional geometries can beused, such as a trapezoidal, elliptical, or rectangular. Also, althoughnot shown in the drawings, the corners and edges may be rounded orotherwise irregularly shaped.

As indicated above, an optical signal confined within the strip loadedwaveguide 200 can be coupled from or is coupled to other opticalcomponents, such as for example modulators, switches, and detectors, atwaveguide input ports and the waveguide output ports. These opticalcomponents may be waveguide structures having the features describedabove. Such configurations allow for further processing or transmissionof the optical signal.

Furthermore, multiple strip loaded waveguides can be positioned atopeach other on the substrate, thereby forming a layered integrated opticstructure. Accordingly, a plurality of strip loaded waveguides can becombined into a system comprising waveguide networks, thus allowingoptical signals to be coupled between components. The specifications ofsuch alternate configurations may be determined by the particularapplication in which the strip loaded waveguide structure is to be used.

Advantageously, the specific material systems that can be used toimplement these strip loaded waveguides have numerous desirablefeatures. Single crystal silicon and polycrystalline silicon aresubstantially transparent at wavelengths in the near infrared spectrum(i.e., between approximately 1.3 μm and 1.6 μm) and thus provide anefficient medium for the propagation of near infrared light. Thecombination of silicon (crystalline or polysilicon) and silicon dioxidealso possesses a high refractive index contrast, i.e., the differencebetween the refractive index of the materials is relatively large. Inparticular, the index of refraction of crystalline silicon andpolysilicon is about 3.5 depending on a variety of parameters. Incontrast, silicon dioxide has an index of refraction of about 1.5. Thisdisparity in refractive index between silicon and silicon dioxide isapproximately 2.0, and is large in comparison for example with thedisparity in refractive index between the silica core and silicacladding that make up conventional optical fiber, both of which areabout 1.5. The difference between the refractive indices of the core andcladding in silica based fiber is approximately 0.003. Thiscore/cladding index difference in the strip loaded waveguides describedabove that comprise silicon and silicon dioxide are approximately threeorders of magnitude higher than that of silica optical fiber. In otherembodiments, the core/cladding index difference is preferably at leastabout 1.0. High index contrast is advantageous because it providesincreased optical confinement of the light within the waveguide.Accordingly, high index contrast allows waveguides having substantiallysmaller dimensions to be employed. Additionally, sharper bends andsmaller bend radii can be incorporated into the waveguides with outexcessive losses.

In addition, certain of the embodiments of the strip loaded waveguidecan be fabricated using conventional integrated circuit fabricationprocesses. For instance, the supporting structure 220 may comprise acommercially available silicon wafer with silicon dioxide formedthereon. Conventional “Silicon-on Oxide” (SOI) processes can be employedto form the silicon slab 205 on a silicon wafer or on a sapphiresubstrate. Fabrication techniques for forming the a crystal siliconlayer on an insulator include, but are not limited to, bonding thecrystal silicon on oxide, SIMOX (i.e., use of ion implantation to formoxide in a region of single crystal silicon), or growing silicon onsapphire. Oxide formation on the silicon slab can be achieved withconventional techniques for growing gate oxides on a silicon activelayers in field effect transistors (FETs). Still other processesutilized in fabricating FETs can also be applied. In the same fashionthat a polysilicon gate is formed on the gate oxide in field effecttransistors, likewise, a polysilicon strip can be formed over the oxidetransition region in the strip loaded waveguide. This polysilicon stripcan be patterned using well-known techniques such as photolithographyand etching. Damascene processes are also considered possible.Accordingly, conventional processes such as those employed in thefabrication of Complementary Metal Oxide Semiconductor (CMOS)transistors can be used to create the waveguide. In other embodiments,crystalline silicon strips can be formed on the transition oxide regionusing conventional techniques such as SOI processing.

Another processing advantage is that in the fabrication of polysiliconor silicon strips 210, the transition layer 215 that separates the slab205 from the strip 210 may in some cases act as an etch stop. Forexample, in applications where the strip 210 and the slab 205 are etchedfrom the same material, the etch can be configured to stop on the thintransition layer 215 therebetween. This fabrication configuration allowsthe geometry of the waveguide to be accurately controlled without havingto dynamically control the etch depth.

Another strategy for fabricating the strip loaded waveguide is to obtaina commercially available SOI wafer which comprises a first siliconsubstrate having a first silicon dioxide layer thereon with a secondlayer of silicon on the first silicon dioxide layer. The aggregatestructure therefore corresponds to Si/SiO₂/Si. The first silicon dioxidelayer is also referred to as the buried oxide or BOX. A second silicondioxide layer can be formed on the SOI wafer and polysilicon or siliconstrips 210 can be formed on this structure to create strip loadedwaveguides 200 with the second silicon layer corresponding to the slab205 and the second silicon dioxide layer formed thereon corresponding tothe transition layer 215. The thickness of this second silicon dioxidetransition layer can be controlled as needed. The polysilicon or siliconstrips can be patterned for example using photolithography and etching.Damascene processes are also envisioned as possible.

In the case where the substrate does not comprise silicon (with a layerof silicon dioxide on the surface), a slab comprising crystal siliconcan still be fabricated. For example, crystalline silicon can be grownon sapphire. The sapphire will serve as the lower cladding for the slab.Silicon nitride formed for example on silicon can also be a cladding forthe slab. The formation of the transition layer and the strip on thesilicon slab can be performed in a manner as described above.

Other conventional processes for forming layers and patterning may alsobe used and are not limited to those specifically recited herein.Employing conventional processes well known in the art is advantageousbecause the performance of these processes is well established. SOI andCMOS fabrication processes, for example, are well developed and welltested, and are capable of reliably producing high quality products. Thehigh precision and small feature size possible with these processesshould theoretically apply to fabrication of strip-loaded waveguides asthe material systems are similar. Accordingly, extremely small sizedwaveguide structures and components should be realizable, therebyenabling a large number of such waveguides and other components to beintegrated on a single die. Although conventional processes can beemployed to form the strip loaded waveguides described herein, andmoreover, one of the distinct advantages is that conventionalsemiconductor fabrication processes can readily be used, the fabricationprocesses should not be limited to only those currently known in art.Other processes yet to be discovered or developed are also considered aspossibly being useful in the formation of these structures.

Another advantage of these designs is that in various embodimentselectronics, such as transistors, can be fabricated on the samesubstrate as the strip loaded waveguides. Additionally, integration ofwaveguides and electronics on the same substrate is particularlyadvantageous because many systems require the functionality offered byboth electronic, optical, electro-optical, and optoelectroniccomponents. For example, with the waveguide structures describe herein,modulators, switches, and detectors, can be optically connected togetherin a network of waveguides and electrically connected to control anddata processing circuitry all on the same die. The integration of thesedifferent components on a single die is particularly advantageous infacilitating minimization of the size of devices, such as opticaltelecommunications devices.

The integration of integrated optical components and with electronics ona single die is illustrated in FIG. 5, which depicts a cross-sectionalview of a strip loaded waveguide 300 disposed on a substrate 320 thatalso supports a field effect transistor 350. As discussed above, thissubstrate 320 may comprise a silicon wafer having a silicon dioxidesurface layer, or a sapphire substrate. A silicon layer 305 is formed onthe silicon substrate 320, and more particularly on the silicon dioxidesurface layer of the substrate. This silicon layer 305 corresponds bothto the slab of the strip loaded waveguide 300 and the active silicon ofthe transistor 350. Accordingly, both the slab and the active silicon ofthe transistor 350 where the channel is formed preferably comprise thesame material and substantially the same thickness although thethicknesses may vary in some embodiments. Both may comprise a dopedsemiconductor. The localized doping concentrations may vary slightly asthe transistor will include source, drain and channel regions withdifferent doping than that of the remainder of the semiconductor layer.

A thin oxide layer 315 is formed on the silicon layer 305. This thinoxide layer 315 corresponds to the transition layer of the strip loadedwaveguide 300 and the gate oxide of the field effect transistor 350.Accordingly, the transition layer of the strip loaded waveguide 300 andthe gate oxide of the FET 350 preferably comprise the same material andpreferably have substantially the same thickness although thethicknesses may vary in some embodiments.

A patterned polysilicon layer 310 can be formed on the thin oxide layer315. This patterned polysilicon layer 310 includes both the strip on thestrip loaded waveguide 300 and the gate on the field effect transistor350. In other embodiments, the gate of the transistor comprises singlecrystal silicon. Likewise, the strip of the strip loaded waveguide 300and the gate of the transistor 350 preferably comprise the same materialand have substantially the same thickness although the thicknesses mayvary in some embodiments. The strip, however, may be an elongatedstructure to facilitate the propagation of light along a pathway fromone location to another on the integrated optical chip. Likewise, thispolysilicon or crystal silicon strip may turn and bend, and split or becombined with other strips. In contrast, the transistor gate ispreferably not elongated and may be more square than the strip (as seenfrom the top, i.e., in a plane parallel to the x-z plane shown in thedrawings). The shapes of the strips are not restricted to square or evenrectangle (as seen from a top) as bends and turns and splitting andcombining as well as intersections may be included among the manyfunctionalities of the waveguides. Additionally, transistors often usesalicides to enhance conductivity at ohmic contacts. In contrast, unlesselectrical connections are to be formed on the waveguides, the waveguidestructure preferably does not include salicides so as to reduceabsorption losses.

Advantageously, in such embodiments the strip loaded waveguide 300 andthe transistor 350 can be fabricated using the same fabricationprocesses. For example, the same substrate may be employed. The slab 305of the waveguide 300 and the active silicon of the transistor 350 can beformed by the same silicon growth, deposition or other formationprocess. Similarly, the transition layer 315 and the gate oxide can begrown or formed in the same processing step. The strip 310 and gate canbe created both by patterning polysilicon (or crystal silicon) at thesame stage of the process. Accordingly, substantially the samefabrication processes can be used to produce both the transistors andthe waveguides. In fact, these structures can be realized substantiallysimultaneously.

In the fabrication of certain semiconductor electronics, it may bedesired to provide spacers such as, for example, silicon nitridespacers. In particular, gate spacers positioned adjacent to the gate ofthe field-effect transistor (“FET”) prevent unwanted doping below thegate. This unwanted doping may result from ion implantation employed todope source and drain regions adjacent the gate. In embodiments whereinstrip loaded waveguides and electronic components are formed on the samesubstrate using the same fabrication process, it will often be desirableto fabricate gate spacers on both the strip loaded waveguides as well asthe electronic components although such spacers can be included evenwhen the transistors are not present on the chip.

FIG. 6 illustrates one preferred embodiment of a strip loaded waveguide500 having spacers 545. The strip loaded waveguide 500 comprises a slab505, a strip 510, and a transition layer 515 therebetween. The striploaded waveguide 500 is disposed on substrate 520 which may include adielectric layer corresponding to the lower cladding of the strip loadedwaveguide 500. Spacers (e.g., gate spacers) 545 are fabricated adjacentto the strip 510. The spacers 545 may comprise a nitride or an oxide,although other preferably nonconductive materials can be used in otherembodiments. In addition to preventing ion doping in regions proximal tothe gate layer in transistors, in certain circumstance, the spacers 545may prevent doping in the region beneath strip. The spacers can also beused to alter the effective index in the slab and to thereby adjust theconfinement within the guiding region and/or to prevent salicide fromforming near the waveguide.

FIG. 6 also shows liners 550 between the spacers 545 and the strip 510.These liners 550 may comprise, for example silicon dioxide, and may beused as passivation for the strip or gate 510. The liners may also actas etch-stop layers. In alternative embodiments, the liners 550 may notbe present, and the spacers 545 may be in direct contact with the strip510.

In various embodiments, the index of refraction of the strip loadedwaveguide can be actively controlled with an applied field. FIG. 7illustrates such a configuration wherein a voltage can be applied acrossa strip loaded waveguide 400. The strip loaded waveguide 400 includes aslab 405, preferably comprising crystalline silicon, and a strip 410,preferably comprising polysilicon or crystalline silicon disposed on asubstrate 420. The silicon slab 405 and the poly or silicon strip 410are preferably doped so as to be conductive. As described above, a thintransition layer 415, comprising for example gate oxide such as silicondioxide, separates the strip 410 and the slab 405. A dielectric coating430, which may be formed from multiple layers, covers the strip 410 andslab 405 and provides electrical insulation. Conductive plugs 445 withinthe dielectric provide a substantially conductive pathways to the polyor silicon strip 410 and the silicon slab 405. Salicide or metalization460, and/or ohmic contacts 440, can be formed on or in the polysiliconor silicon strip 410 or the silicon slab 405 to electrically couple theplugs 445 to these portions of the strip loaded waveguide 400. A voltagesource 435 is electrically connected to the plugs 445.

Application of a voltage between the polysilicon or silicon strip 410and the silicon slab 405 causes carriers 450 to accumulate within theguiding region 425 of the strip loaded waveguide 400. For example,depending on the applied voltage, its polarity, and the doping of thestrip 410 and the slab 405, electrons or holes may accumulated or bedepleted within the strip 410 or the slab 405 in regions adjacent to thethin transition layer 415 comprising gate oxide. The structure acts likea capacitor, charging with application of a voltage. The voltage createsan electric field across the thin transition layer 415 with carriers 450accumulating (or depleting) adjacent to this transition layer 415.Preferably, the transition layer 415 is sufficiently thick such that thecarriers do not traverse this barrier layer by tunneling or throughdefects, such as pinhole defects. Conversely, the thickness of thisdielectric layer 415 is preferably not so large as to require a largevoltage to be applied to the device to generate enough carriers to varythe index of the strip loaded waveguide 400. The thickness of this layerwill also be affected by similar considerations in transistors formed onthe same layer as the strip loaded waveguide 400. For example, in fieldeffect transistors, the gate oxide is preferably sufficiently thick soas to prevent tunneling of carriers from the channel region into thegate but is sufficiently thin such that the voltage required to activatethe transistor is not too large.

The magnitude of the applied voltage and the resultant electric fieldacross the transition layer 415 controls the carrier density of thestrip loaded waveguide 400. Preferably, the carrier density at leastwithin the guiding region 425 is altered by the application of thevoltage. This carrier accumulation or depletion may be concentratedpredominately in the strip 410 or the portion of the slab 405 beneaththe strip 410. The refractive index of semiconductor material alterswith variation in carrier concentration. The accumulation of carrierslowers the index of refraction while depletion of carriers raises theindex. The refractive index of the strip 410 and portions of the slab405 can therefore be altered by controlling the carrier density inregions therein. For instance, by accumulating or depleting carriers inthe proximity of the transition layer 415, the effective index of thestrip 410 and the slab 405 can be altered as desired. In addition toaffecting the refractive index, accumulation of carriers also increasesabsorption. Application of a field can therefore also vary theabsorption coefficient associated with the waveguide.

Accordingly, the optical properties of the waveguide 400 may becontrollably altered with application of an electric bias. The index ofrefraction can be varied to alter the effective optical path distancewithin the guide and adjust or tune the guide for different wavelengths,introduce or reduce phase delay, and increase or decrease opticalconfinement within the guide, or to otherwise affect the lightpropagating within the guide as desired. Since the absorption can alsobe controlled, the intensity of the light can be altered. Electronicbiasing therefore can be employed to the modulate the signal or tocreate other optical or electro-optical components which can be operatedby actively changing the index of the refraction and/or the absorptionof the waveguide or portions of it. Electronic biasing can also be usedto adjust or tune waveguide structures to account for, e.g.,manufacturing tolerances, or to configure the structure for differentapplications.

Gate spacers (not shown in FIG. 7) may further be included as discussedabove and may minimize fringing of the electric field in the case wherea dielectric coating 230 does not cover the strip loaded waveguide.

In an alternative configuration illustrated in FIG. 8, an additionalpoly or silicon layer 470 can be formed over the strip 410 with adielectric region 475 separating this additional poly or silicon layerand the strip. Electrical connection may be made to this additional polyor silicon layer 470 and to the strip 410 via metalization 460 on theadditional poly or silicon layer and conducting plugs 445 though thedielectric 430 to the metalization. A conductive pathway is alsoprovided to the strip 410 by way of metal plugs 445 and an ohmic contact440 in the strip. Application of a voltage between the additional polyor silicon layer 470 and the strip 410 will cause carriers 450 toaccumulate or be depleted in the strip 410. This arrangement enables thecarrier density of the strip 410 to be altered independent of thecarrier density within the slab 405. Accordingly, the index ofrefraction and/or absorption can be changed predominantly within thestrip 410, while these properties in the slab 405 are preferablyunaltered. Other configurations suited to the particular application areconsidered possible. For example, electrically connection can also bemade with the slab 405 and a voltage can be applied between the strip410 and the slab to alter the carrier distribution below the strip andaffect the index of refraction in the slab.

In each of these designs, regardless of whether the waveguide isconfigured for application of an electronic field, the properties of thesemiconductor portions can be adjusted based on how the material isdoped with impurities, if any.

Also, as discussed above with reference to FIG. 2, the dimensions of thestrip 210 and the slab 205 may vary depending on the application of thewaveguide. For example, in an application wherein the waveguide must beconfigured to propagate only a single optical mode, the dimensions ofthe strip 210 (and possibly the slab 205) may be adjusted accordingly.The dimensions of the strip portion 210 and the slab portion 205 mayalso depend on the wavelength of the optical signal confined in thewaveguide.

In certain embodiments, the dimensions of the strip loaded waveguide 210can be selected such that only a single mode and single polarization canbe propagated in the guiding region 225. These special strip loadedwaveguides are single mode waveguides that in addition only support onepolarization. In one example, for instance, the dimensions of thewaveguide can be designed so as to support only the transverse-electric(“TE”) fundamental mode. The TE mode corresponds to light having apolarization parallel to the interface between the slab 205 andtransition layer 215 or the strip 210 and the transition layer 215 (thatis, with the electric field is parallel to the x-z plane as defined inFIG. 2). For light having a wavelength of 1.55 μm, single TE modeoperation can be obtained by configuring the thickness of the slabportion 205 to be approximately 110 nm, the thickness of the stripportion 210 to be approximately 95 nm, and the thickness of thetransition portion 215 to be approximately 40 nm. The strip 210 has awidth of about 0.5 micrometers. Finite difference time domain iterationsand eigenmode solvers can be used to determine appropriate dimensionsfor other such strip loaded waveguides that supports a single TE mode.In this particular case, the slab portion 205 and the strip portion 210both comprise single crystal silicon, and the transition portion 215comprises silicon dioxide. However, specific embodiments with differentmaterials and different dimensions can be obtained that support only asingle polarization mode. Such a configuration may be particularlyadvantageous in certain polarization-dependent applications where onlyone polarization is required. Such a waveguide, for example, can act asa linear polarizer. These waveguides that support a single polarizationof the fundamental mode may also be employed to minimize crosstalk.

In alternative embodiments, as illustrated in FIG. 9, the strip loadedwaveguide 600 may comprise a strip 610 formed directly on a slab 605that is supported by substrate 620. In such embodiments, no low-indextransition layer is positioned between the strip 610 and the slab 605.The presence of the strip 610 positioned adjacent to the slab 605induces an increase in effective index of the slab portion 605 in theregion directly under the strip 610 and in proximity thereto. Thisincrease in effective index defines a relatively high effective indexguiding region 625 wherein light in one or more supported optical modesis guided along the strip loaded waveguide 600. This strip 610 comprisespolysilicon and the slab 605 comprises crystal silicon. The crystalsilicon slab 605 may be formed on a oxide or nitride layer on a siliconsubstrate. Other insulator layers may be employed as the lower claddinglayer and as the substrate. For example, sapphire may be used as asubstrate with crystal silicon formed thereon. One or more layers oflower index material such as glass or oxide may be formed over the strip610 and the slab 605.

As described above, silicon is substantially optically transmissive tocertain wavelengths of interest such as 1.55 microns. In addition,processes for silicon fabricating such structures are well developed.For these reasons, a waveguide comprising polysilicon and silicon isadvantageous.

Although silicon is beneficial because it is substantially transparentat certain wavelengths, other materials and more particularly, othersemiconductors may be employed. Furthermore, the structures describedherein are not to be limited to any particular wavelength or wavelengthrange and may be designed, for example, for microwave, infrared,visible, and ultraviolet wavelengths.

Those skilled in the art will appreciate that the methods and designsdescribed above have additional applications and that the relevantapplications are not limited to those specifically recited above. Also,the present invention may be embodied in other specific forms withoutdeparting from the essential characteristics as described herein. Theembodiments described above are to be considered in all respects asillustrative only and not restrictive in any manner.

What is claimed is:
 1. A silicon-based strip loaded waveguidecomprising: a slab portion comprising doped silicon, the slab portionhaving a first refractive index (n₁); a strip portion comprising asilicon-based material selected from the group consisting essentially ofdoped crystal silicon and polysilicon, the strip portion having a secondrefractive index (n₂); and an insulating transition portion disposedbetween the slab portion and the strip portion, the insulatingtransition comprising a silicon-based dielectric material, theinsulating transition portion having a third refractive index (n₃) thatis less than the first refractive index (n₁) and the second refractiveindex (n₂), said silicon-based dielectric material in said insulatingtransition portion insulating said slab portion from said strip portion,wherein said strip portion locally increases the effective index of saidslab portion so as to guide light in said slab proximal to said stripportion thereby supporting guided optical propagation in both said stripportion and said slab portion.
 2. The waveguide of claim 1, wherein thetransition portion comprises silicon dioxide.
 3. The waveguide of claim1, wherein the slab portion comprises single crystal silicon.
 4. Thewaveguide of claim 1, wherein the strip portion comprises polysilicon.5. The waveguide of claim 1, wherein the strip portion comprises singlecrystal silicon.
 6. The waveguide of claim 1, wherein the slab portionand the strip portion comprise substantially the same material.
 7. Thewaveguide of claim 1, further comprising a substrate that supports saidslab and said strip.
 8. The waveguide of claim 7, wherein the substratecomprises a silicon wafer.
 9. The waveguide of claim 7, wherein thesubstrate comprises sapphire.
 10. The waveguide of claim 1, furthercomprising a lower cladding layer adjacent said slab, said lowercladding layer having a refractive index less than the first refractiveindex (n₁) of said slab.
 11. The waveguide of claim 10, wherein thedifference between first refractive index (n₁) of said slab and saidrefractive index of said lower cladding layer is at least about
 1. 12.The waveguide of claim 11, wherein the difference between firstrefractive index (n₁) of said slab and said refractive index of saidlower cladding layer is at least about
 2. 13. The waveguide of claim 10,wherein said lower cladding layer comprises a layer of silicon dioxide.14. The waveguide of claim 10, wherein said lower cladding layercomprises a layer of silicon nitride.
 15. The waveguide of claim 10,further comprising an upper cladding adjacent said slab and said strip,said upper cladding has a refractive index that is less than both thefirst refractive index (n₁) and second refractive index (n₂).
 16. Thewaveguide of claim 15, wherein the difference between a refractive indexof said upper cladding and said first refractive index (n₁) of said slaband said second refractive index (n₂) of said strip is at least about 1.17. The waveguide of claim 16, wherein the difference between arefractive index of said upper cladding and said first refractive index(n₁) of said slab and said second refractive index (n₂) of said strip isat least about
 2. 18. The waveguide of claim 15, wherein the uppercladding comprises glass.
 19. The waveguide of claim, 15, wherein theupper cladding comprises silicon dioxide.
 20. The waveguide of claim 15,wherein the upper cladding and the lower cladding comprise substantiallythe same material.
 21. The waveguide of claim 1, wherein said strip hasa width and thickness, and said slab and transition regions haverespective thicknesses so as to support a fundamental optical mode witha cross-sectional power distribution profile having two intensity maximaconnected by an intensity minima, one of said intensity maxima locatedin said strip portions, one of said intensity maxima located in saidslab portion, and said intensity minima located in said insulatingtransition portion.
 22. A strip loaded single-mode waveguide comprisingsilicon for propagating light having a wavelength, said strip loadedsingle-mode waveguide comprising: a slab portion comprising materialcomprising silicon; a dielectric transition portion comprisingdielectric material disposed over said slab portion; and a strip portioncomprising a material comprising silicon, said strip portion disposedover said dielectric transition portion and with respect to the slabportion to form a guiding region, a first portion of the guiding regionbeing in the strip portion, a second portion of said guiding regionbeing in said dielectric transition portion, and a third portion of theguiding region being in the slab portion, the guiding region configuredto propagate light of the wavelength only in a single spatial mode andonly in a single polarization, said single polarization corresponding toa transverse electric (TE) mode, said single spatial mode having twoseparated intensity peaks, one of said intensity peaks in said siliconstrip portion and one of said intensity peaks in said silicon slabportion.
 23. The strip loaded single-mode waveguide of claim 22, whereinsaid slab portion and strip portion comprise single crystal silicon. 24.The strip loaded single-mode waveguide of claim 23, wherein said slabportion has a thickness of approximately 90 nm and said strip portionhas a thickness of approximately 40 nm.
 25. The strip loaded single-modewaveguide of claim 22, wherein the strip portion comprises polysilicon.26. A high index contrast strip loaded waveguide comprising siliconmaterial, said high index contrast strip loaded waveguide comprising: aslab portion comprising silicon; a strip portion comprisingsilicon-based material selected from the group consisting essentially ofcrystal silicon and polysilicon, said strip portion disposed withrespect to the slab portion to form a guiding region, a first portion ofthe guiding region being in the strip portion, and a second portion ofthe guiding region being in the slab portion; and a dielectric layerdisposed between said slab portion and said strip portion, saiddielectric comprising silicon-based material, said guiding regionextending through said dielectric layer, wherein the guiding regionpropagates light in a single spatial mode with a cross-sectional powerdistribution profile having two intensity maxima, one of which islocated in the slab portion and the other of which is located in stripportion.
 27. The strip loaded waveguide of claim 26, wherein saiddielectric layer comprises silicon dioxide.
 28. The strip loadedwaveguide of claim 26, further comprising upper and lower claddinglayers above said strip portion and below said slab portionrespectively.
 29. The strip loaded waveguide of claim 28, wherein saidlower cladding layer comprise silicon dioxide.
 30. A high index contraststrip loaded waveguide having a guiding region for guiding light throughthe waveguide, the guiding region comprising a strip comprising a layerof polycrystalline silicon juxtaposed with a slab comprising a layer ofcrystal silicon, said polysilicon strip and said crystal silicon slabsurrounded by cladding, said polycrystalline string locally increasingthe effective index of said crystal silicon slab so as to guide light insaid crystal silicon slab proximal to said polysilicon strip, saidguiding region thereby supporting guided optical propagation in bothsaid polycrystalline silicon strip and said crystal silicon slab. 31.The waveguide of claim 30, wherein said strip has a width and athickness and said slab has a thickness to support only a singlefundamental optical mode to the exclusion of higher order modes.
 32. Ahigh index contrast strip loaded waveguide comprising: a slab portioncomprising silicon and having a first refractive index (n₁); a stripportion comprising a silicon having a second refractive index (n₂); aninsulating transition portion comprising a dielectric material betweenthe slab portion and the strip portion, the transition portion having athird refractive index (n₃) that is less than the first refractive index(n₁) of the slab portion and the second refractive index (n₂) of thestrip portion, said dielectric material in said insulating transitionportion insulating said slab portion from said strip portion; and acladding for confining light within said strip portion and slab portion,said cladding having a fourth refractive index (n₄) that is less thanthe first refractive index (n₁) of the slab portion and the secondrefractive index (n₂) of the strip portion, the difference between saidfourth refractive index (n₄) of the cladding and said first refractiveindex (n₁) of the slab portion being at least about 1.0.
 33. Thewaveguide of claim 32, wherein the difference between fourth refractiveindex (n₄) of said cladding and said first refractive index of the slabportion is at least about 2.